(i) Field of the Invention
This invention relates generally to a hydraulic control system for automatic transmissions, and in particular, to a control system which suppresses shift shocks.
(ii) Description of the Prior Art
The speed change gear system which shifts an automatic transmission to and from a high speed range and a low speed range generally comprises planetary gear sets and friction coupling devices which releasably couple the component parts of the planetary gear sets with stationary or rotational members in the automatic transmission case. The transmission is shifted by selectively coupling the component parts of the planetary gear set with the stationary or rotational members.
In a case where an automatic transmission with the above-described speed change gear system is not provided with a one-way clutch or other means for mechanically controlling the shifts between the high and low speed ranges, it has been the conventional practice to provide an orifice in the drain conduit of a hydraulic servo which establishes the low speed range, varying the diameter of the orifice to adjust the drain speed of the oil from the hydraulic servo for adjusting the timing of engagement and release of friction elements at the time of shifts. However, it has been difficult to control the timing appropriately. In this connection, FIG. 10 is a diagram of a case where the drain orifice in the drain conduit for the low speed range has a small diameter at the time of a shift from a low to high speed range, while FIG. 11 shows a case where the drain orifice has a large diameter. (x=the oil pressure of the hydraulic servo for the high speed range, and y=the oil pressure of the hydraulic servo for the low speed range.) In FIG. 10, the friction elements of both the high and low speed ranges are engaged during the time period from Tc to Td, so that the output shaft is held stationary, abruptly braking the vehicle and therefore impairing the shift feeling. In the case of FIG. 11, the friction elements of both the high and low speed ranges are disengaged during the time period of from Tc to Tb, increasing the rotational speed of the engine. Accordingly, there may occur overrunning of the engine in a high speed operation with a large throttle opening. Further, the increase of the engine speed during the time period from Tc to Tb invites accumulation of an increased amount of rotational energy in rotating parts (e.g., engine, drive plate, transmission etc.), so that a larger amount of energy is discharged upon engagement of the friction element of the high speed range, giving a greater shock at the time of a shift.